“Items collection must be empty before using ItemsSource.” error wpf

Question

I am trying to bind an observable collection of strings. But when i launch an app, I receive Exception that Items collection must be empty before using ItemsSource. I have no elements in collection when it is binding, so what can be the issue?

My Xaml

<ListBox ItemsSource="{Binding Users}"  Margin="10,77,805,228" Grid.RowSpan="2">
                    <ListBoxItem>
                        <DataTemplate>
                            <StackPanel Orientation="Horizontal">

                            </StackPanel>
                        </DataTemplate>
                    </ListBoxItem>
                </ListBox>
<Button x:Name="AddUserButton" Content="Додати" Command="{Binding AddUserCommand}" RenderTransformOrigin="0.512,1.9"  />

My ViewModel (command and observablecollection)

public class UsersTabViewModel : ViewModelBase
{
    private ObservableCollection<string> users;
    private string text;

    private ICommand addUserCommand;

    private bool _canExecute;

    public UsersTabViewModel() 
    {
        _canExecute = true;
        Users = new ObservableCollection<string>();
    }

    public ObservableCollection<string> Users { get; set; }


    public ICommand AddUserCommand
    {
        get
        {
            return addUserCommand ?? (addUserCommand = new CommandHandler(() => AddUserAction(), _canExecute)); 
        }

    }

    public string Text
    {
        get
        {
            return text;
        }

        set
        {
            text = value;
        }
    } 

    //text is bound to here
    private void AddUserAction()
    {

        Users.Add("collection");


    }

    public class CommandHandler : ICommand
    {
        private Action _action;
        private bool _canExecute;
        public CommandHandler(Action action, bool canExecute)
        {
            _action = action;
            _canExecute = canExecute;
        }

        public bool CanExecute(object parameter)
        {
            return _canExecute;
        }

        public event EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            _action();
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

SLaks SLaks · Accepted Answer · 2016-12-23T19:59:07

As the error is trying to tell you, you can't have any items if you use ItemsSource to bind them instead.
Remove your <ListBoxItem>.

To set a template for the bound items, set <ListBox.ItemTemplate>.

“Items collection must be empty before using ItemsSource.” error wpf

2 Answers